What are impaired waters?
http://www.epa.gov/region9/water/tmdl/303d.html
The waterbody is assessed for all of its “beneficial uses.” Then quantitative or narravtive standards are established for each beneficial use. “Impaired” waters are those that do not meet the established standards.
For instance, in my watershed, salmon need certain temperature water to thrive. We don’t achieve that, so we are “impaired.” It does not seem to matter that the temperature in summer here exceeds 100 degreesF.